A Triassic Cuddle Set in Stone

In 1975, near the base of South Africa’s Oliviershoek Pass, paleontologist James Kitching discovered the final resting place of a small, shuffling mammal that had perished some 250 million years before. Little more than a piece of skull was poking out of the stone, but the shape and composition of the surrounding rock suggested that the poor creature had died in a burrow – and that there might be more inside. Sure enough, when Kitching cracked open the rock the little lair was pocked by even more bones, so off it went to the collection of Johannesburg’s Evolutionary Studies Institute of the University of Witwatersrand. Kitching had no idea that he had found a pair of unusual Triassic bedfellows.

The part of the fossil Kitching first spotted was a piece of Thrinaxodon. Multiple specimens of this small, squat protomammal have been found curled up inside burrows. Whether or not Thrinaxodon made their own dens is a mystery, but their remains, fossilized in repose, hint that they escaped the blistering heat of the dry season by snoozing underground.

Thrinaxodon is not alone in the Triassic tomb. Lying belly-up atop the protomammal is a rare, salamander-like amphibian named Broomistega. No one had any idea that the bonus fossil was there until University of the Witwatersrand paleontologist Vincent Fernandez and colleagues had the contents of the burrow scanned at the European Synchrotron Radiation Facility in Grenoble, France. Presented in the scan’s digital detail, published last year, the pair of fossils rest against each other in stunning articulation.

How did this Triassic mash-up come together? The fossil doesn’t offer a definite answer, but Fernandez and colleagues narrowed down the list of possibilities.

Regardless of whether the Thrinaxodon created the hollow, previous fossil finds and the anatomical improbability of a burrowing Broomistega suggest that the protomammal was the den’s primary occupant. And even though both animals were buried by a mix of water and sediment that sluiced into the burrow, it would be a hell of a coincidence if the sloshing mud carried an intact amphibian right into the burrow.

With such an accidental burial unlikely, Broomistega was either dragged in by Thrinaxodon or the amphibian purposely hauled itself into the burrow. The latter seems more likely. Even though the Broomistega bones show two possible tooth puncture marks, the size and spacing did not match the dental particulars of the Thrinaxodon. Instead, Fernandez and colleagues hypothesized, the Broomistega simply wandered into the burrow where the Thrinaxodon was sound asleep in a brief torpor, and there the amphibian lay until a mucky slurry buried them both.

Such close cohabitation is rare, even among modern animals, but the Broomistega may have had good reasons to seek shelter. For one thing, this particular animal had a series of broken, partly-healed ribs that probably hindered the amphibian’s ability to move and breathe. That’s a major problem for a creature that will quickly die if stranded in dry season sun, so perhaps the burrow was the closest place of refuge for the injured Broomistega. So long as the Thrinaxodon lay undisturbed in a multi-day slumber, as Fernandez and coauthors suspect, then the amphibian could have rested in the cool without risk of being run out by the burrow-owner’s snapping jaws. In the cool and the dark, the Triassic neighbors dozed together, died together, and became fossilized together.

Reference:

Fernandez, V., Abdala, F., Carlson, K., Cook ,D., Rubidge, B., Yates, A., Tafforeau, P. 2013. Synchrotron reveals Early Triassic odd couple: Injured amphibian and aestivating therapsid share burrow. PLoS ONE 8, 6: e64978. doi:10.1371/journal.pone.0064978